Communication satellites operating in the Ka-band frequency range often use a large number of microwave communication beams. The satellites typically have a lifetime of over 15 years. It is rather difficult to predict the information carrying capacity of individual beams during such a long period of time. Accordingly, dynamic allocation of the bandwidth among the microwave communication beams is highly desirable.
The dynamic bandwidth allocation can be provided by tunable microwave filters having a tunable central frequency and a variable width of the passband. Such tunable microwave filters can be installed in both the ground stations and onboard the satellites. To be practically usable, the tunable filters must possess a high stability of the spectral response, strong out-of-band rejection, and small group delay variation. Furthermore, tunable filters placed on satellites must be lightweight and meet stringent space-launch qualification requirements.
Electronic filters are ubiquitous in circuit design. Many types of electronic filters are presently available. The most common filters use bulk elements, such as capacitors, inductors, and so on, to form single- or multi-pole filters at appropriate frequencies of interest. It is possible to build tunable filters using switching banks or variable components. Although these filters can operate up to several gigahertz in frequency, they are mostly used for lower frequencies, where the physical dimensions of the components are still small compared to the wavelength of operation.
Circuits operating at microwave frequencies typically use planar or coaxial waveguide structures. These filters utilize distributed capacitance and inductance created by a particular geometry of the waveguide structure, in conjunction with the abrupt variations in impedance created by stubs and slots, to form resonant cavities. Using ceramic materials or high-temperature superconductors to form very low loss substrates can result in very high finesse (high-Q) filters. Planar structures are reasonably easy to fabricate using conventional circuit board techniques. However, the circuit board based planar structures tend to be lossy at higher frequencies due to radiative loss. Coaxial structures are superior in this regard because the outer conductor shields the structure, but these tend to be bulky and heavy.
Millimeter-wave filters can be formed using dielectric resonators and cavity structures, but they are difficult to fabricate, and the resulting filter characteristics can be very sensitive to fabrication errors, particularly when the filters contain multiple coupled resonators. Both the microwave and the millimeter-wave filters are difficult to tune and have a limited tuning range. Furthermore, it is difficult to change the finesse of a particular filter or to generate a variable bandwidth filter.
In a satellite, a bank of filters is switched in and out of a signal path to change the channel bandwidth, and a programmable frequency converter is used to change the center frequency. There are two major difficulties associated with this approach. First, a very limited number of filters can be practically used due to a large number of communication beams, and even these few filters per beam result in a very heavy and bulky overall structure. Second, once the set of filters is determined, it remains fixed for the lifetime of the satellite. Because of these intrinsic difficulties, other approaches have been investigated.
One such approach, presented by Ming Yu el al. in a paper entitled “A Ka Band Tunable Filter for Reconfigurable Payload”, 15th Ka and Broadband Communications, Navigation and Earth Observation Conference, Sep. 23-25, 2009, which is incorporated herein by reference, consists of having a mechanically tunable cavity filter. However, any mechanically controlled devices or subsystems in a satellite raise substantial reliability issues.
Another approach, exemplified in a paper by Glyn Thomas et al. entitled “Agile Equipment for an Advanced Ku/Ka Satellite”, ESA Workshop on Advanced Flexible Telecom Payloads, 18-20 Nov. 2008, ESA/ESTEC, Noordwijk The Netherlands, which is incorporated herein by reference, uses an electrical heterodyne principle. The signal is frequency down-converted to a given intermediate frequency (IF) using a programmable synthesizer, two cascaded bandpass filters are used to achieve the required filtering, and then another programmable synthesizer is used to bring the signal to the desired channel frequency. The main drawbacks of this circuit are the power consumption, and large volume and mass, which are all very detrimental for a space application.
Electrical filters based on photonic circuits have been reported numerous times, primarily in the academic literature. These are generally based on: tapped delay lines to emulate a finite impulse response (FIR) filter, delay line interferometers, fiber Bragg grating (FBG) delay lines, dispersive fiber delays, and acousto-optic modulators. In general, these techniques are better suited towards forming notch filters, not bandpass filters required for a satellite bandwidth allocation and tuning applications.
Ilchenko et al. disclose in United States Patent Application US2005/0175358, which is incorporated herein by reference, a tunable radio frequency and microwave photonic filter using an optical heterodyne principle. Referring to FIG. 1, a filter 100 of Ilchenko et al. is shown having a laser 101, an electro-optical modulator (EOM) 102, a whispering-gallery mode (WGM) filter 103, a photodetector 104, beamsplitters 105, and mirrors 106. The WGM filter 103 has evanescent field couplers 107 and cascaded WGM resonators 108. In operation, the laser 101 emits a beam at a carrier frequency that is modulated by the EOM 102 with a radio frequency input signal 110 to create sidelobes in a spectrum of the optical signal. The WGM filter 103 selects one such sidelobe. A fraction of the laser beam is split by the beamsplitter 105 before the EOM 102 to propagate through a path 109 defined by the beamsplitters 105 and the mirrors 106. The photodetector 104 receives the combined modulated and the split laser beam and provides an output electrical signal 111 at a differential frequency between the passband frequency of the filter 103 and the carrier frequency. By tuning the WGM filter 103, the passband central frequency of the filter 100 can be tuned.
The filter of Ilchenko et al. suffers from the drawbacks of overall complexity and lack of stability due to presence of multiple optical elements and optical paths.
Accordingly, it is a goal of the present invention to provide a filter of a millimeter-wave or microwave signal, which would be lightweight, simple, reliable, and tunable in both central frequency and bandwidth.